This invention relates generally to molded case circuit breakers and more particularly to a current transformer for the solid state trip unit of the circuit breaker.
The use of modern circuit breakers with electronic trip systems is increasing. An obvious reason for this is the inherent versatility and precise repeatability of selected trip time response characteristics which are an improvement over the prior thermal and magnetic trip units. Because of the obvious advantages of using electronic trip units in circuit breakers, these electronic units are being used in smaller circuit breakers to provide these advantages throughout the entire line of circuit breakers.
Part of the reason for the concern about the size of the trip system is that manufacturers are making circuit breakers smaller. In recent years there has been a reduction in the overall size of circuit breakers or to state it more accurately, greater current interrupting capability is now demanded in the same space previously occupied for lower current interrupting ratings. The end result is that circuit breakers are smaller.
While it is possible to take some steps to reduce the size of electronic trip systems, one of the limiting factors in the size reduction of electronic trip units is the current transformer. Typically, there is one current transformer per phase in the trip unit; thus, in a three-phase system, there will be at least three current transformers. At the lower end of the circuit breaker line, in order to fit electronic trip systems into the small space allocated, the current transformers must typically be no wider than about an inch.
Typical current transformers used in circuit breakers are illustrated in U.S. Pat. No. 4,181,389 which issued on Jan. 1, 1980 to G. W. Kiesel and H. G. Willard, U.S. Pat. No. 3,761,776 which issued on Sept. 25, 1973 to H. G. Willard and U.S. Pat. No. 4,281,359 which issued on July 28, 1981 to E. W. Bayer and E. A. Palmisano. These patents illustrate current transformer arrangements wherein the transformer coil is encoiled about the transformer core with a bus or current carrying member extending through the core in such a fashion that current flows through the center of the transformer core to set up the appropriate magnetic flux. Unfortunately, the bus adds to the width of the current transformer requiring that the dimensions of the trip unit be increased to accommodate the bus thereby making the bus a limiting factor in the size reduction of the current transformer and the trip unit. Accordingly, it will be appreciated that it will be highly desirable to provide a current transformer in which the bus is not a limiting factor in the size of the electronic trip unit.
Accordingly, it is an object of the present invention to provide a current transformer for a circuit breaker.
Another object of the present invention is to provide a bus arrangement for a current transformer which does not increase the width of the tranformer.